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The oomycetes rarely have septa (see hypha), and if they do, they are scarce,[6] appearing at the bases of sporangia, and sometimes in older parts of the filaments.[7] Some are unicellular, but others are filamentous and branching.[7]

This group was originally classified among the fungi (the name "oomycota" means "egg fungus") and later treated as protists, based on general morphology and lifestyle.[5] A cladistic analysis based on modern discoveries about the biology of these organisms supports a relatively close relationship with some photosynthetic organisms such as brown algae and diatoms. A common taxonomic classification based on these data, places the class Oomycota along with other classes such as Phaeophyceae (brown algae) within the phylumHeterokonta.

This relationship is supported by a number of observed differences in the characteristics of oomycetes and fungi. For instance, the cell walls of oomycetes are composed of cellulose rather than chitin[8] and generally do not have septations. Also, in the vegetative state they have diploid nuclei, whereas fungi have haploid nuclei. Most oomycetes produce self-motile zoospores with two flagella. One flagellum has a "whiplash" morphology, and the other a branched "tinsel" morphology. The "tinsel" flagellum is unique to the Kingdom Heterokonta. Spores of the few fungal groups which retain flagella (such as the Chytridiomycetes) have only one whiplash flagellum.[8] Oomycota and fungi have different metabolic pathways for synthesizing lysine and have a number of enzymes which differ.[8] The ultrastructure is also different, with oomycota having tubular mitochondrial cristae and fungi having flattened cristae.[8]

In spite of this, many species of oomycetes are still described or listed as types of fungi and may sometimes be referred to as pseudofungi, or lower fungi.

"Oomycota" means "egg fungi", referring to the large round oogonia, structures containing the female gametes, that are characteristic of the oomycetes.

The name "water mold" refers to their earlier classification as fungi and their preference for conditions of high humidity and running surface water, which is characteristic for the basal taxa of the oomycetes.

Most of the oomycetes produce two distinct types of spores. The main dispersive spores are asexual, self-motile spores called zoospores, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind. They also produce sexual spores, called oospores, that are translucent, double-walled, spherical structures used to survive adverse environmental conditions.

Many oomycetes species are economically important, aggressive plant pathogens. Some species can cause disease in fish. The majority of the plant pathogenic species can be classified into four groups, although more exist.

The paraphyleticPythium group is more prevalent than Phytophthora and individual species have larger host ranges, although usually causing less damage. Pythiumdamping off is a very common problem in greenhouses, where the organism kills newly emerged seedlings. Mycoparasitic members of this group (e.g. P. oligandrum) parasitize other oomycetes and fungi, and have been employed as biocontrol agents. One Pythium species, Pythium insidiosum, is also known to infect mammals.

The third group are the downy mildews, which are easily identifiable by the appearance of white, brownish or olive "mildew" on the leaf undersides (although this group can be confused with the unrelated fungal powdery mildews).

^Kortekamp, A. (2005). "Growth, occurrence and development of septa in Plasmopara viticola and other members of the Peronosporaceae using light- and epifluorescence-microscopy". Mycological research109 (Pt 5): 640–648. doi:10.1017/S0953756205002418. PMID16018320.edit